Diffuse optical tomography, also referred to as optical tomographic imaging, is often performed by measuring the amplitude attenuation of light that has been passed through tissue. Propagation of injected photons is determined by the spatially-varying absorption and scattering characteristics of the tissue being probed. In biological tissues, scattering interactions are often the principal mechanisms affecting the light trajectory. As a result of this highly scattering nature, these photons do not navigate in a straight procession but rather diffuse throughout the medium. The photon flux exiting the tissue at any single point is the net effect of the incident light source, and discrete absorption and scattering interactions throughout their pathlength.
By illuminating several locations around the tissue of interest, and detecting transmitted and back-reflected intensities at multiple positions along the surface, one can generate tomographic images, similar to X-ray computed tomography. The transmitted intensity measured along the target surface maintains the same frequency with respect to the source. However, the measured intensity will exhibit an amplitude attenuation and an induced phase shift. This amplitude attenuation and phase shift provide spatial information regarding the absorption and scattering distribution inside the tissue.
In continuous wave imaging, the light is either illuminated at a constant amplitude or modulated by a low frequency sine wave (up to a few kHz), and the decay in amplitude relative to the incident source is measured. If the illuminated source is modulated, then a synchronous or homodyne detection technique is often employed to extract the zero-frequency amplitude information. This method requires generating a reference signal whose frequency is equal to and phase locked with the input waveform. Multiplying the input waveform by its reference signal produces an output waveform that is a composite of two independent contributions; one component located at zero-frequency and the other component straddling twice the modulation frequency.
This resulting mixed signal is then sent through a low-pass filter to eliminate the higher frequency component, leaving only the remaining DC constituent whose amplitude is directly proportional to the amplitude of the detected optical signal. By imaging with multiple wavelengths, the spectral information is increased allowing investigators to formulate qualitative assessments of hemoglobin parameters such as oxyhemoglobin and deoxyhemoglobin, or quantitative valuations of additional physiologic chromophores. Each wavelength must be modulated at distinct frequencies and/or phase in order to isolate the individual signals and their respective amplitudes.
Almost all continuous-wave optical tomography systems currently cited in literature perform any relevant signal conditioning and data processing through analog techniques, such as described in C. H. Schmitz, M. Locker, J. M. Lasker, A. H. Hielscher, R. L. Barbour, “Instrumentation for fast functional optical tomography,” Review of Scientific Instruments, Vol. 73, pp. 429-439 (2002), which is hereby incorporated by reference herein in its entirety (which publication is hereinafter referred to as “Schmitz 2002”). Analog systems are used to collect, condition, and possibly filter the incident signal. For those instruments that modulate the intensity of their light source, analog phase-sensitive lock-in methods are usually employed to extricate the optical signal obscured by noise of potentially greater magnitude.
Such analog detection systems, however, suffer from a number of deficiencies and limitations. More specifically, for example, analog phase-sensitive detection has many problems associated with it that adversely affect their performance and restrict subsequent applications. Some primary deficiencies include, for example, signal drift, output offsets, gain error, limited dynamic reserve, and harmonic rejection. Additionally, external parameters such as temperature or age contribute to analog noise and, consequently, measurement uncertainty. Furthermore, analog processing is notably sensitive to component tolerances thereby limiting functional utility.
Finally, when the digital timing signals share a backplane with analog data signals, coupling can occur, causing fluctuations along the analog lines. A direct consequence of these undesirable attributes is that the instrument noise floor is elevated, causing a reduction in the detection sensitivity, diminished dynamic range for the overall system, and a slowing of the data acquisition. Analog detection systems suffer from other deficiencies and limitations, as well.
U.S. Pat. No. 7,463,362 to Lasker et al. (Lasker) discloses digital signal processor-based detection systems and methods for optical tomography to address some of the deficiencies and performance limitations of existing analog-based systems. However, there are still challenges to be addressed for digital optical tomographic imaging systems, such as system expansion (including the number of detectors), timing considerations, acquisition speed, and clinically-practical user interface design.